This invention generally relates to digital projection apparatus employing liquid crystal devices for image forming and more particularly to an apparatus and method for achieving high levels of contrast by using a slightly rotated wire grid polarization beamsplitter in combination with a liquid crystal display (LCD) and a polarization compensator for minimizing leakage light in the pixel black (OFF) state.
In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. In particular, to provide a competitive alternative to conventional cinematic-quality projectors, digital projection systems need to provide high resolution, wide color gamut, high brightness ( greater than 10,000 screen lumens), and frame-sequential system contrast ratios exceeding 1,000:1. In addition, the digital systems must also provide constancy of image quality, image data security, low equipment purchase and maintenance costs, and low data distribution costs, to make a switchover from conventional film based systems compelling.
The most promising solutions for digital cinema projection employ one of two types of spatial light modulators as image forming devices. The first type of spatial light modulator is the digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, including for example U.S. Pat. Nos. 4,441,791 and 5,600,383 (both to Hornbeck). Optical designs for projection apparatus employing DMDs are disclosed in numerous patents, including U.S. Pat. Nos. 5,914,818 (Tejada et al.) and 6,089,717 (Iwai). Although DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, the current resolution limitations (1024xc3x97768 pixels), as well as high component and system costs, have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is the liquid crystal device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. At high resolution, large area LCDs can be fabricated more readily than DMDs. LCDs are a viable alternative modulator technology to be used in digital cinema projection systems. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. Nos. 5,808,795 (Shimomura et al.) and 5,918,961 (Ueda). A few years ago, JVC demonstrated an LCD-based projector capable of high-resolution (providing 2,000xc3x971280 pixels), high frame sequential contrast (in excess of 1000:1), and high light throughput (nominally, up to 12,000 lumens). This system utilized three vertically aligned (VA) (also referred as homeotropic) LCDs (one per color) driven or addressed by cathode ray tubes (CRTs). While this system demonstrated the potential for an LCD based digital cinema projector, system complexity and overall reliability remain concerns. In addition, that particular prototype system had a high unit cost that made it unacceptable for broad commercialization in a digital cinema projection market.
JVC has also developed a new family of vertically aligned LCDs, which are directly addressed via a silicon backplane (LCOS), rather than indirectly by a CRT. While these new devices are promising, they have not yet been demonstrated to fully meet the expectations for digital cinema presentation. The JVC LCD devices are described, in part, in U.S. Pat. Nos. 5,652,667 (Kuragane) and 5,978,056 (Shintani et al.) In contrast to most twisted nematic or cholesteric LCDs, vertically aligned LCDs promise to provide much higher modulation contrast ratios (in excess of 2,000:1). It is instructive to note that, in order to obtain on screen frame sequential contrast of 1,000:1 or better, the entire system must produce  greater than 1000:1 contrast, and both the LCDs and any necessary polarization optics must each separately provide xcx9c2,000:1 contrast. Notably, while polarization compensated vertically aligned LCDs can provide contrast  greater than 20,000:1 when modulating collimated laser beams, these same modulators may exhibit contrasts of 500:1 or less when modulating the same collimated laser beams without the appropriate polarization compensation. Modulation contrast is also dependent on the spectral bandwidth and angular width (F#) of the incident light, with contrast generally dropping as the bandwidth is increased or the F# is decreased. Modulation contrast within LCDs can also be reduced by residual depolarization or mis-orienting polarization effects, such as thermally induced stress birefringence. Such effects can be observed in the far field of the device, where the ideally observed xe2x80x9ciron crossxe2x80x9d polarization contrast pattern takes on a degenerate pattern.
As is obvious to those skilled in the digital projection art, the optical performance provided by a LCD based electronic projection system is, in large part, defined by the characteristics of the LCDs themselves and by the polarization optics that support LCD projection. The performance of polarization separation optics, such as polarization beamsplitters, pre-polarizers, and polarizer/analyzer components is of particular importance for obtaining high contrast ratios. The precise manner in which these polarization optical components are combined within a modulation optical system of a projection display can also have significant impact on the final resultant contrast.
The most common conventional polarization beamsplitter solution, which is used in many projection systems, is the traditional MacNeille prism, disclosed in U.S. Pat. No. 2,403,731. This device has been shown to provide a good extinction ratio (on the order of 300:1). However, this standard prism operates well only with incident light over a limited range of angles (a few degrees). Because the MacNeille prism design provides good extinction ratio for one polarization state only, a design using this device must effectively discard half of the incoming light when this light is from an unpolarized white light source, such as from a xenon or metal halide arc lamp.
Conventional glass polarization beamsplitter design, based on the MacNeille design, has other limitations beyond the limited angular response, including fabrication or thermally induced stress birefringence. These effects, which can degrade the polarization contrast performance, may be acceptable for mid range electronic projection applications, but are not tolerable for cinema projection applications. The thermal stress problem has been improved upon, with the use of a more suitable low photo-elasticity optical glass, disclosed in U.S. Pat. No. 5,969,861 (Ueda et al.), which was specially designed for use in polarization components. Unfortunately, high fabrication costs and uncertain availability limit the utility of this solution. As a result of these problems, the conventional MacNeille based glass beamsplitter design, which works for low to mid-range electronic projection systems, operating at 500-5,000 lumens with approximately 800:1 contrast, falls short for digital cinema projection.
Other polarization beamsplitter technologies have been proposed to meet the needs of a LCD based digital cinema projection system. For example, the beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li et al.) has theoretical transmitted and reflected extinction ratios in excess of 2,000:1. This prism offers the potential of using both polarizations with a six LCD system, thereby enhancing system light efficiency. However, size constraints and extremely tight coating tolerances present significant obstacles to commercialization of a projection apparatus using this beamsplitter design.
Alternately, liquid-filled beamsplitters (see U.S. Pat. No. 5,844,722 (Stephens), for example) have been shown to provide high extinction ratios needed for high-contrast applications and have some advantages under high-intensity light conditions. However, these devices have several operational problems including temperature sensitivity, are costly to manufacture, and must be fabricated without dust or contained bubbles. Leakage risk presents another potential disadvantage for these devices.
Wire grid polarizers have been in existence for many years, and were primarily used in radio-frequency and far infrared optical applications. Use of wire grid polarizers with visible spectrum light has been limited, largely due to constraints of device performance or manufacture. For example, U.S. Pat. No. 5,383,053 (Hegg et al.) discloses use of a wire grid beamsplitter in a virtual image display apparatus, which has high light efficiency but very low contrast (6.3:1). A second wire grid polarizer for the visible spectrum is disclosed in U.S. Pat. No. 5,748,368 (Tamada). While the device discussed by Tamada provides polarization separation, the contrast ratio is inadequate for cinematic projection and the design is inherently limited to rather narrow wavelength bands.
Recently, as is disclosed in U.S. Pat. Nos. 6,122,103 (Perkins et al.); 6,243,199 (Hansen et al.); and 6,288,840 (Perkins et al.), high quality wire grid polarizers and beamsplitters have been developed for broadband use in the visible spectrum. These new devices are commercially available from Moxtek Inc. of Orem, Utah. While existing wire grid polarizers, including the devices described in U.S. Pat. Nos. 6,122,103 and 6,243,199 may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices do have a number of advantages. When compared against standard polarizers, wire grid polarization devices exhibit relatively high extinction ratios and high efficiency. Additionally, the contrast performance of these wire grid devices also has broader angular acceptance (NA or numerical aperture) and more robust thermal performance with less opportunity for thermally induced stress birefringence than standard polarization devices. Furthermore, the wire grid polarizers are robust relative to harsh environmental conditions, such as light intensity, temperature, and vibration. While generally these commercially available wire grid devices perform well across the visible spectrum, an innate blue fall off in the polarization response can mean that the blue channel may require additional contrast enhancement to match the red and green for demanding applications.
Wire grid polarization beamsplitter (PBS) devices have been employed within some digital projection apparatus. For example, U.S. Pat. No. 6,243,199 (Hansen et al.) discloses use of a broadband wire grid polarization beamsplitter for projection display applications. U.S. Pat. No. 6,234,634 (also to Hansen et al.) discloses a wire grid polarization beamsplitter that functions as both polarizer and analyzer in a digital image projection system. U.S. Pat. No. 6,234,634 states that very low effective F#""s can be achieved using wire grid PBS, although with some loss of contrast. Notably, U.S. Pat. No. 6,234,634 does not discuss how the angular response of the wire grid polarizers can be enhanced, nor how polarization compensation may be used in combination with wire grid devices and LCDs, to reduce light leakage and boost contrast, particularly for fast optical systems operating at low F#""s.
Of particular interest and relevance for the apparatus and methods of the present invention, it must be emphasized that individually neither the wire grid polarizer, nor the wire grid polarization beamsplitter, provide the target polarization extinction ratio performance (nominally  greater than 2,000:1) needed to achieve the desired projection system frame sequential contrast of 1,000:1 or better, particularly at small F#""s ( less than F/3.5). Rather, both of these components provide less than xcx9c1,200:1 contrast under the best conditions. Significantly, performance falls off further in the blue spectrum. Therefore, to achieve the desired 2,000:1 contrast target for the optical portion of the system (excluding the LCDs), it is necessary to utilize a variety of polarization devices, including possibly wire grid polarization devices, in combination within a modulation optical system of the projection display. However, the issues of designing an optimized configuration of polarization optics, including wire grid polarizers and polarization compensators, in combination with LCDs, color optics, and projection lens, have not been completely addressed either for electronic projection in general, or for digital cinema projection in particular. Moreover, the prior art does not describe how to design a modulation optical system for a projection display using both LCDs and wire grid devices, which further has polarization compensators to boost contrast.
There are numerous examples of polarization compensators developed to enhance the polarization performance of LCDs generally, and vertically aligned LCDs particularly. In an optimized system, the compensators are simultaneously designed to enhance the performance of the LCDs and the polarization optics in combination. These compensators typically provide angularly varying birefringence, structured in a spatially variant fashion, to affect polarization states in portions (within certain spatial and angular areas) of the transiting light beam, without affecting the polarization states in other portions of the light beam. As a first example, U.S. Pat. No. 4,701,028 (Clerc et al.) discloses birefringence compensation designed for a vertically aligned LCD with restricted thickness. As another example, U.S. Pat. No. 5,039,185 (Uchida et al.) discloses a vertically aligned LCD with compensator comprising at least two uniaxial or two biaxial retarders provided between a sheet polarizer/analyzer pair. Additionally, U.S. Pat. No. 5,298,199 (Hirose et al.) discloses the use of a biaxial film compensator correcting for optical birefringence errors in the LCD, used in a package with crossed sheet polarizers, where the LCD dark state has a non-zero voltage (a bias voltage).
Compensators can be complex structures, comprising one or more layers of films, optical adhesives, and other materials. For example, U.S. Pat. No. 5,619,352 (Koch et al.) discloses compensation devices, usable with twisted nematic LCDs, where the compensators have a multi-layer construction, using combinations of A-plates, C-plates, and O-plates, as needed.
Polarization compensators can also be designed which correct for both the vertically aligned LCD and the polarization optics in combination. Most of these prior art compensator patents discussed previously, assume the LCDs are used in combination with sheet polarizers, and correct only for the LCD polarization errors. However, polarization compensators have also been explicitly developed to correct for non-uniform polarization effects from the conventional Polaroid type dye sheet polarizer. The dye sheet polarizer, developed by E. H. Land in 1929 functions by dichroism, or the polarization selective anistropic absorption of light. Compensators for dye sheet polarizers are described in Chen et al. (J. Chen, K.-H. Kim, J.-J. Kyu, J. H. Souk, J. R. Kelly, P. J. Bos, xe2x80x9cOptimum Film Compensation Modes for TN and VA LCDsxe2x80x9d, SID 98 Digest, pgs. 315-318.), and use a combination A-plate and C-plate construction. Similarly, U.S. Pat. No. 5,576,854 (Schmidt et al.) discloses a compensator constructed for use in projector apparatus using an LCD with the conventional MacNeille prism type polarization beamsplitter. This compensator comprises a xc2xc wave plate for compensating the prism and an additional 0.02xcex""s compensation for the inherent LCD residual birefringence effects.
While this prior art material extensively details the design of polarization compensators used under various conditions, compensators explicitly developed and optimized for use with wire grid polarizers and vertically aligned LCDs are not disclosed in the prior art. In order to achieve high brightness levels, it is most advantageous for an optical system to have a high numerical aperture ( greater than xcx9c0.13), so that it is able to gather incident light at larger oblique angles. The conflicting goals of maintaining high brightness and high contrast ratio present a significant design problem for polarization components. Light leakage in the OFF state must be minimal in order to achieve high contrast levels. Yet, light leakage is most pronounced for incident light at the oblique angles required for achieving high brightness.
However, as is disclosed in commonly assigned co-pending U.S. patent application Ser. No. 10/040,663, polarization compensators have been developed and optimized for wire grid polarizers and polarization beam splitters. In particular, this application describes compensators designed for the wire grid devices, as well as compensators for wire grid devices which also work with vertically aligned LCDs and with compensators for vertically aligned LCDs. It has been shown that a modulation optical system comprising wire grid polarizers, a wire grid polarization beamsplitter, a vertically aligned LCD, and a customized polarization compensator, can provide polarization contrast in excess of the 1,000:1 target across a wide range of incident angles (small F""s).
However, the fabrication of the polarization compensators used in such a system can be difficult, as, depending on the compensator design, specific values and orientations of retardance are required, and are assembled from a combination of existing materials. Typically these materials are thin film sheets, such as polycarbonate or acetate, whose optical retardance depends both on material properties and film fabrication methods. Compensators can then be assembled by stacking an appropriate combination of these films between glass plates, with intervening layers of optical adhesive to provide optical index matching. The assembled compensator must be free from both dirt and bubbles, and provide a consistent spatially uniform retardance while under a large heat (light) load. Alternately, a compensation layer with a nominal target retardance can be spun coated directly on a glass substrate, thereby potentially simplifying the construction of the compensator device. However, the construction of compensators that require multiple retardation layers with different properties can still be difficult. Furthermore, the optimum retardance required to correct for the inherent residual birefringence (such as the 0.02xcex""s mentioned previously) can vary significantly from device to device. Ideally, but likely impractically, this implies that to maximize contrast from device to device would require matching each LCD with an appropriately optimized compensator.
Given these various difficulties in providing robust uniform polarization compensators that maximize the polarization response of both the polarizers and the LCDs, it is evident that a design for a modulation optical system that simplifies the use of these compensator is an improvement. In general, the prior art does not describe how to design and optimize a modulation optical system for a projection display using both LCDs and wire grid polarization devices, which further has polarization compensators to boost contrast. Therefore, it can be seen that there is a need for an improved projection apparatus that uses wire grid polarization devices, vertically aligned LCDs, and polarization compensators in combination to provide high-contrast output. In particular, this invention will describe a modulation optical system, which can be used within projection display systems, printing systems, or for other applications, in which the wire grid polarizers are rotated slightly, in order to introduce retardance, and thus tune the performance of the LCD and polarization compensator, or simplify the design and construction of the polarization compensator, or provide a substitute for the polarization compensator, as depends on the design details of a given system.
Briefly, according to one aspect of the present invention a display apparatus comprises a light source for forming a beam of light. A pre-polarizer polarizes the beam of light to provide a polarized beam of light. A wire grid polarization beamsplitter receives the polarized beam of light and transmits the polarized beam of light which has a first polarization, and reflects the polarized beam of light which has a second polarization. A reflective spatial light modulator selectively modulates the polarized beam of light that has a first polarization to encode image data thereon in order to form a modulated beam and reflects the modulated beam back to the wire grid polarization beamsplitter. A compensator is located between the wire grid polarization beamsplitter and the reflective spatial light modulator for conditioning oblique and skew rays of the modulated beam. The wire grid polarization beamsplitter reflects the compensated modulated beam and the wire gird polarization beamsplitter is rotated in plane to optimize the contrast. A polarization analyzer removes residual light of the opposite polarization state from the compensated modulated beam. Image-forming optics form an image from the compensated modulated beam.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.